The campaign to persuade Gordon Brown to hold a referendum on the new EU treaty is gaining force. The drums are rolling, time is short and this choice cannot be fudged. Immediately after next month's EU summit meeting, he and his colleagues have to decide whether to accept the treaty and ram it through the Commons, using his personal authority, or risk a crisis in relations with other European leaders, and the future of the treaty, by throwing caution to the winds and agreeing that the people have a right to vote directly.

Fighting for a referendum are "usual suspect" Labour MPs, including Gisela Stuart, Frank Field and Kate Hoey; the still Eurosceptic Conservatives, led on this issue by that arch-Brusselsphobe William Hague; and now some of the big voices in the trade unions, led by the GMB and the RMT. Their motives are mixed and, to the foreign secretary David Miliband, suspect too.

It's easy to see why the prime minister may not want a referendum. Surely, here is the moment to show himself a committed European and to reassert the powers of parliament, by pushing it all through the Commons. Yet to my own surprise, the more I look at the proposed new treaty, the more I find myself in the referendum camp. They may be a strange alliance, from rightwing newspaper types to anti-Brown leftists, from dissident Labour MPs to the UK Independence party. But those calling for a national choice are absolutely right and, if they are listened to, will strengthen not weaken the government. Brown and Miliband need to think very carefully before trying to bolt the door on them.

Let's start with the basics. Brown's case to the country has been that he offers a genuinely fresh start, a "new politics" that eschews deviousness in favour of plain dealing. His strong performance in the polls is based on people hoping that's true, plus his highly competent early moves. He has won sceptics round - a bit - by talking plainly, by sticking with early promises to tell parliament first about new policies and by working through cabinet. But it is all fragile, and there are millions of people who haven't quite made up their minds about him. Lose them, and he'll lose the election.

The government promised a referendum on the original treaty. That caused much irritation in Brussels, and yet even the very pro-European Tony Blair decided it was the right thing to do. Only the revolt of French and Dutch voters got him off the hook. The big problem for the government now is that, to avoid accusations of bad faith, it would have to show that the new treaty is significantly less important than the old one. And at the moment, that isn't true. Apart from trivial changes, such as the use of symbols and music, it is overwhelmingly similar. Many of the key figures in Europe actually celebrate this.

Listen to Germany's Angela Merkel: "The substance of the constitution is preserved. That is a fact." What about Valéry Giscard d'Estaing, the aristocratic former French president who was in charge of the original treaty? He told a London conference that although Britain, France and the Netherlands had demanded the word "constitution" be eliminated, all the key elements were still there: "All the earlier proposals will be in the new text but will be hidden or disguised in some way." Spain's prime minister José Zapatero? "We have not let a single substantial point of the constitutional treaty go."

There are plenty of other examples from across the EU. And if you judge the treaty by the core where-does-power-lie questions, they are surely right. The change in voting includes the end of Britain's veto in 61 areas. The EU court of justice gets new powers over policing and criminal justice. There would be a new EU diplomatic service and someone who would be, in effect, Europe's foreign minister. Though national parliaments do get very limited new rights to scrutinise what the Brussels commission proposes, they are weak and balanced by a formal insistence that national parliaments should actively contribute "to the good functioning of the union".

Now, as it happens, I approve of much of this. To make the larger EU function better, we do have to give up some veto powers, for instance; and after the mayhem of the last US-dominated decade, it would be a good thing if the EU exercised more influence in world affairs. But what I can't manage to do is to pretend that all of the above is somehow unimportant, and does not add up to a new constitutional treaty. So, a question: how likely is it that the British people, who seem to be overwhelmingly in favour of a referendum, will be persuaded despite the known facts, and the cheerfully unhelpful contributions of Merkel, Giscard, Zapatero and all, that this is a mere tidying-up exercise that can be left to parliament?

Not very. It is true that there are British opt-outs. But as the former Europe minister Keith Vaz has pointed out, they are the same ones as in the original treaty. Nor are they unequivocal. Vaz, like David Blunkett and other mainstream MPs, has concluded that the argument against a referendum is lost. And, to cap it all, John Hutton has promised that "if there is some significant constitutional arrangement that would affect our relationship with the European Union ... there should be a proper referendum."

So Brown is under huge pressure, and not just from stroppy union leaders who call the new treaty "anti-labour to the core". Brown has a reputation as moderately Eurosceptic, based mainly on his hostility to joining the euro, which has won him newspaper support but is likely to cause him increasing difficulty on the continent. Accepting a referendum would not be risk-free. Other leaders would be furious. At home, it would unleash hostility from hostile Blairites who have so far bitten their tongues.

Above all, consider the alternative. Parliament doesn't actually get the right to properly scrutinise the treaty, since parliament can't amend it and send it back for second thoughts. So the idea is that in the teeth of public hostility, and on the back of a threadbare, widely ridiculed argument, he should ram this through Westminster? Not only would that be wrong in principle, it would severely damage Brown's hard-won reputation for straight dealing and give the Tories the opening they've been searching for. Come on, Gordon.

Losing a referendum would be a political embarrassment. Refusing one would be a political disaster.